Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers. Generally speaking, a power amplifier amplifies an audio signal by taking energy from a power supply and controlling an audio output signal to match an input signal shape but with a larger amplitude.
One example of an audio amplifier is a class-D amplifier. A class-D amplifier (also known as a “switching amplifier”) may comprise an electronic amplifier in which the amplifying devices (e.g., transistors, typically metal-oxide-semiconductor field effect transistors) operate as electronic switches. In a class-D amplifier, a signal to be amplified may be converted to a series of pulses by pulse-width modulation, pulse-density modulation, or another method of modulation, such that the signal is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) is a function of the magnitude of the signal. After amplification with a class-D amplifier, the output pulse train may be converted to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in the class-D amplifier or a load driven by the class-D amplifier. Class-D amplifiers are often used due to the fact that they may be more power efficient than linear analog amplifiers, in that class-D amplifiers may dissipate less power as heat in active devices as compared to linear analog amplifiers.
Typically, a closed-loop PWM amplifier is chosen in order to provide accurate load voltage with desirable Total Harmonic Distortion (THD) and Power Supply Rejection Ratio (PSRR). A closed-loop PWM amplifier typically takes an analog voltage input and a sensed feedback voltage signal which are fed through a closed-loop analog PWM modulator to drive voltage on the speaker load.
However, an option to alternatively drive loads using a single PWM amplifier circuit in either an open-loop mode (e.g. using a class-D open-loop driver) or a closed-loop mode (e.g., using a closed-loop driver) depending on the specific application may be desirable. The open-loop mode may advantageously consume less power than the closed-loop mode, with the disadvantage that signal distortion in the open-loop mode may increase as signal magnitude of the signal to be amplified by the open-loop path increases. Thus, as a signal envelope increases, it may be desirable to switch to the closed-loop mode due to the high distortion levels in the open-loop path, with the trade-off of higher power consumption in the closed-loop path.
When using such a single PWM amplifier circuit, perceptible audio artifacts may occur when switching between open-loop operation and closed-loop operation, and thus, it may be desirable to reduce or eliminate such audio artifacts.